X‐ray photoelectron spectroscopy ‐An introductionAn XPS spectrum consists of peaks corresponding...

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1 X‐ray photoelectron spectroscopy ‐ An introduction Spyros Diplas [email protected] [email protected] SINTEF Industry, Materials Physics -Oslo & Centre of Materials Science and Nanotechnology, Department of Chemistry, UiO

Transcript of X‐ray photoelectron spectroscopy ‐An introductionAn XPS spectrum consists of peaks corresponding...

Page 1: X‐ray photoelectron spectroscopy ‐An introductionAn XPS spectrum consists of peaks corresponding to emission of both photoelectrons and Auger electrons. 6 Auger electron vs x‐ray

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X‐ray photoelectron spectroscopy ‐ An introduction 

Spyros Diplas

[email protected]

[email protected]

SINTEF Industry, Materials Physics -Oslo&

Centre of Materials Science and Nanotechnology, Department of Chemistry, UiO

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Material Characterisation Methods

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What is surface?• What happens at surfaces is extremely important in a vast range of applications from environmental corrosion to 

medical implants.

• A surface is really the interface between different phases (solid, liquid or gas).

• We can think of the surface as the top layer of atoms but in reality the state of this layer is very much influenced by the 2 – 10 atomic layers below it (~0.5 – 3 nm).

• Surface modification treatments are often in the range of 10 – 100 nm thick.  >100 nm can be thought of as the bulk.

• Surface analysis encompasses techniques which probe the properties in all these ranges.

God made solids, but surfaces were the work of the devil ------Wolfgang Pauli

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• Properties and reactivity of the surface will depend on:• bonding geometry of molecules to the surface  • physical topography• chemical composition• chemical structure• atomic structure• electronic state

No one technique can provide all these pieces of information.  However, to solve a specific problem it is seldom necessary to use every technique available.

photons

ions

electrons

EMISSION

TRANSMISSION

Interaction with material

EXCITATION

Surface Analysis ‐ Techniques Available

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Photoelectron

2p1/2, 2p3/2

2s

1s

Ekin = hν – EB -

L23

L1

K

EKL2,3L2,3(Z) = EK(Z) – [EL2,3(Z) + EL2,3(Z + 1)]

Internal transition

(irradiative)

Auger electron

XPS‐Basic Principle

valence bandFermi

Vacuum

De-excitationExcitationAn XPS spectrum consists of peaks corresponding to emission of both photoelectrons and Auger electrons

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Auger electron vs x‐ray emission yield

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B Ne P Ca Mn Zn Br Zr

10 15 20 25 30 35 40 Atomic Number

Elemental Symbol

0

0.2

0.4

0.6

0.8

1.0Pr

obab

ility

Auger  Electron Emission

X‐ray Photon Emission

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Schematic of an XPS spectrometer

Number of emitted electrons measured as function of their kinetic energy

Al

X-ray source

Electrostatic electron lens Electron

detector

Electron energy analyser

Samplee-

Photon

Slit

Hemispherical electrodes

Slit

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Examples of XPS spectrometers

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Instrument: Kratos Axis UltraDLD at MiNaLab

Analyser

Monochromator

Sample

Detector

X-ray source

X-ray source

e-

e-

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Why is XPS surface sensitive?XPS Depth of Analysis

The probability that a photoelectron will escape from the sample without losing energy is regulated by the Beer‐Lambert law:

Where λe is the photoelectron inelastic mean free path 

Attenuation length (λ) ≈0.9 IMFPIMFP: The average distance an electron with a given energy travels between

successive inelastic collisions

Typical electron energies in the XPS spectrum

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Features of the XPS spectrum Primary structure

- Core level photoelectron peaks (atom excitation)- Valence band spectra - CCC, CCV, CVV Auger peaks (atom de-excitation)

Secondary structure

- X-ray satellites and ghosts- Shake up and shake off satellites- Plasmon loss features- Background (slope)

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XPS spectrum ITO

x 104

10

20

30

40

50

60

70

80CPS

1200 1000 800 600 400 200 0Binding Energy (eV)

In 3dSn 3d

O 1s

In 3pSn 3p

In 3sIn 3s

In MNNSn MNN

O KLL

Auger peaks

Photoelectron peaks

In/Sn 4pIn/Sn 4s

C 1s

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Peak width (ΔE)ΔE = (ΔEn

2 + ΔEp2 + ΔEa

2)1/2

Gaussian broadening:

-Instrumental:There is no perfectly resolving spectrometer nor a perfectly monochromatic X-ray source.

-Sample:For semiconductor surfaces in particular, variations in the defect density across the surface will lead to variations in the band bendingand, thus, the work function will vary from point to point. This variation in surface potential produces a broadening of the XPS peaks.

-Excitation process such as the shake-up/shake-off processes or vibrational broadening.

Lorentzian broadening:

The core-hole that the incident photon creates has a particular lifetime (τ) which is dependent on how quickly the hole is filled by an electron from another shell. From Heisenberg’s uncertainty principle, the finite lifetime will produce a broadening of the peak.

Γ=h/τ

Intrinsic width of the same energy level should increase with increasing atomic number

Natural width X-ray source contributionAnalyser contribution

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Chemical shift

ΔE(i) =  kΔq +  ΔVM – ΔR

Initial state contribution

• Δq: changes in valence charge

• ΔVM : Coulomb interaction between the photoelectron (i) and the surrounding charged atoms. 

.

final state contribution 

• ΔR: relaxation energy change arising from the response of the atomic environment (local electronic structure) to the screening of the core hole

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Chemical shift - Growth of ITO on p c-Si

104 102 100 98

BHF 15 sec + 500°C

0.5 nm

1.5 nm

Si2p

456 454 452 450 448 446 444 442 440

In3d

0.5 nm

1.5 nm

3.0 nm

500 495 490 485 480

Sn3d

0.5 nm

1.5 nm

3.0 nm

Inte

nsity

arbi

trary

uni

ts

Binding Energy (eV)

SiOx

Si In oxide

In

Sn oxide

Sn

3/2 3/2 5/2 5/2

1.5 nm

0.5 nm

BHF 15 sec + 500oC 0.5 nm

1.5 nm

3.0 nm

0.5 nm

1.5 nm

3.0 nm

Si 2p In 3d Sn 3d

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Chemical shift

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Quantification Unlike AES, SIMS, EDX, WDX there are little in the way of matrix effects to worry about in XPS. We

can use either theoretical or empirical cross sections, corrected for transmission function of the analyser. In principle the following equation can be used:

I = J ρ σ K λ

I is the electron intensity J is the photon flux, ρ is the concentration of the atom or ion in the solid, σ s is the cross-section for photoelectron production (which depends on the element and energy being

considered), K is a term which covers instrumental factors, λ is the electron attenuation length.

In practice atomic sensitivity factors (F) are often used: [A] atomic % = {(IA/FA)/Σ(I/F)} Various compilations are available.

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Spin-Orbit Coupling/Splitting

Spin-orbit coupling/ splitting: final state effect for orbitals with orbital angular momentum l> 0. A magnetic interaction between an electron’s spin and its orbital angular momentum.

Example Ti. Upon photoemission an electron from the p orbital is removed - remaining electron can adopt one of two configurations: a spin-up (s=+1/2) or spin-down (s=-1/2) state. If no spin-orbit interaction these two states would have equal energy (degenerated states).

spin-orbit coupling lifts the degeneracy To realise that we need to consider the quantum number, j, the total angular momentum quantum

number. j=l + s where s is the spin quantum number (±½). For a p orbital j=1/2 or 3/2. Thus the final state of

the system may be either p1/2 or p3/2 and this gives rise to a splitting of the core-level into a doublet as shown in the figure above.

Spin-orbit coupling is described for light elements by the Russell-Saunders (LS) coupling approximation and by the j-j coupling approximation for heavier elements

Arb

itrar

y U

nits

468 466 464 462 460 458 456Binding Energy (eV)

Tioxide 2p 2p3/2

2p1/2

The intensity of the peaks is given by the degeneracy

gJ = 2j+1

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Depth profile with ion sputtering

Use of an ion gun to erode the sample surface and re-analyse Enables layered structures to be investigated Investigations of interfaces Depth resolution improved by:

Low beam energiesSmall ion beam sizesSample rotation

SnO2

Sn

Depth500 496 492 488 484 480

Page 20: X‐ray photoelectron spectroscopy ‐An introductionAn XPS spectrum consists of peaks corresponding to emission of both photoelectrons and Auger electrons. 6 Auger electron vs x‐ray

Elemental distribution and oxygen deficiency of magnetron sputtered ITO filmsA. Thøgersen, M.Rein, E. Monakhov, J. Mayandi, S. Diplas

JOURNAL OF APPLIED PHYSICS 109, 113532 (2011)

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ITO surface

ITO/Si interface

InIn‐oxide

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XPS-Check list Depth of analysis ~ 5nm All elements except H and He Readily quantified (limit ca. 0.1 at%) All materials (vacuum compatible) Chemical/electronic state information

-Identification of chemical states-Reflection of electronic changes to the atomic potential

Compositional depth profiling by -ARXPS (ultra thin film <10 nm), -change of the excitation energy -choose of different spectral areas-sputtering

Ultra thin film thickness measurement Analysis area mm2 to 10 micrometres